Fix pre-existing CI breakage: phpcs violations + phantom functional-tests testsuite

[bonelifer]

Summary

• Fixes trailing whitespace in language/de/overview.php and a missing blank line after the namespace declaration in ext.php and textreparser/plugins/article_text.php. These were failing the phpcs style-check job on every push to main, blocking the whole DB test matrix from ever running.
• Removes phpunit.xml.dist's "Extension Functional Tests" testsuite, which declared ./tests/functional/ — a directory that never existed in this extension. This is the config used for local dev, not by the CI matrix jobs, but it was inaccurate either way.
• Adds tests/functional/.gitkeep. The reusable CI workflow generates each DB matrix job's actual phpunit config by copying phpBB core's own .github/phpunit-*-github.xml template, which always declares a testsuite pointing at ../tests/functional. RUN_FUNCTIONAL_TESTS: 0 only adds --exclude-group functional (a test-group filter) — it doesn't skip declaring that directory, so PHPUnit still requires it to exist before any test runs. Confirmed by reproducing the workflow's exact config-generation step locally and tracing the "Test directory ... not found" error to this. Other phpbbmodders extensions without functional tests instead disable the whole DB matrix (RUN_MYSQL_JOBS: 0, etc.), but this extension has a real DB-backed migration test worth actually running, so this keeps that coverage instead of losing it.

All three are pre-existing and unrelated to any pending feature/docs PR's own content — confirmed by checking main's own CI history, which has been failing on the phpcs step for several commits already.

Test plan

• php -l on all three phpcs-fixed files: no syntax errors.
• Ran the same phpcs command CI uses (--standard=build/code_sniffer/ruleset-php-extensions.xml) against the whole extension: clean, no violations.
• Reproduced the CI workflow's exact sed-based config generation locally against a real phpBB 3.3.x checkout to confirm the root cause of the "Test directory not found" error.
• Could not fully execute PHPUnit locally past config parsing (this environment's PHP 8.4 hits an unrelated, pre-existing backupGlobals incompatibility with this old phpunit.xml schema before reaching directory discovery), so CI itself is the real verification for the last two fixes.

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Investigated and written by Claude on behalf of William Jacoby (bonelifer).